A ball of mass `m` moving with a speed `u_(1)` collides elasticity with another identical ball moving with velocity `u_(2)`.
(a) Find the velocities of the balls after collision if the impact is direct.
(b) Find the angle between velocities after collision if they collide obliquely and `u_(2)=0`

Answer

(a) `v_(1)=u_(2),v_(2)=u_(1)`; (b) `(pi)/(2)`